Precise control of the number of walls formed during carbon nanotube growth using chemical vapor deposition

We demonstrate a one-step approach for selecting the number of walls formed during carbon nanotube (CNT) growth by catalytic decomposition of CH(4) over Fe-Mo/MgO catalysts. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermal gravimetric analysis (TGA) and Raman spectroscopy analyses indicate that high purity single-walled, double-walled and triple-walled carbon nanotubes can be synthesized by tuning the Fe:Mo atomic ratio of catalysts. The results reveal that the concentration of Mo in the catalyst plays an important role in the size of catalyst particles and in the deposition rate of carbon atoms during CNT growth. Thus, the wall numbers of CNTs can be controlled precisely.     

Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by radio frequency-Catalytic Chemical Vapor Deposition (RF-CCVD) through the pyrolysis of CH4. Fe-Co bimetallic catalytic nanoclusters were supported on high-surface area MgO nanopowders and used in the nanotube synthesis process. Nanolog absorption fluorescence analysis was used to characterize the chiralities of the as-produced SWCNTs over this nanostructural catalyst. In the final SWCNT sample, the (7,5) semiconducting carbon nanotube species were found to be dominant, with a low chirality variation.     

Preparation of carbon nanomaterials through CH4 pyrolysis on (Co + Mo)/MgO catalysts with different metal contents

We have studied the influence of active metal content in (Co + Mo)/MgO catalysts (0.5–5 wt % Co + Mo) on the morphology and properties of catalytic methane pyrolysis products (900°C, CH₄: H₂ volume ratio of 1: 1, pyrolysis time in the range 20–120 min). The results demonstrate that, in addition to the main pyrolysis product, carbon nanotubes, the process may yield graphene shells on MgO particles, graphene shells on relatively large Co particles, and amorphous carbon on carbon nanotubes. The metal content of the catalyst determines the relationship between different carbon species, their net yield, and the specific surface area (S) of the products. Increasing the pyrolysis time increases the S of the materials obtained at low metal concentrations in the catalyst, whereas on the catalyst containing 5 wt % metals S remains unchanged. In contrast, the yield of carbon nanomaterials is more sensitive to the pyrolysis time on the catalyst containing 5 wt % metals.     

Catalytic chemical vapor deposition of single-wall carbon nanotubes at low temperatures

We report surface-bound growth of single-wall carbon nanotubes (SWNTs) at temperatures as low as 350 degrees C by catalytic chemical vapor deposition from undiluted C2H2. NH3 or H2 exposure critically facilitates the nanostructuring and activation of sub-nanometer Fe and Al/Fe/Al multilayer catalyst films prior to growth, enabling the SWNT nucleation at lower temperatures. We suggest that carbon nanotube growth is governed by the catalyst surface without the necessity of catalyst liquefaction.     

Carbon Nanotubes Grown by RF Heating and Their Morphological and Structural Properties

Multiwall and single-wall carbon nanotubes were synthesized on Fe-Co/CaCO₃ and a Fe-Co/MgO catalyst system, respectively, by using two different catalytic chemical vapor deposition methods, external furnace (EF) heating and radio frequency (RF) excitation. The carbon nanotubes synthesized with radio frequency excitation have a smaller outer diameter, fewer layers (smaller outer/inner diameter ratio), and better crystalline properties than the nanotubes grown with external furnace heating. The radio frequency process was found to be responsible for a faster growth rate of the carbon nanotubes over longer periods of time due to a higher localized heating. These findings can be explained by the skin currents induced in the metallic catalytic clusters, which keep the catalysts active for longer periods of time and diminish the amount of noncrystalline carbon formed in the synthesis process.     

Antioxidant deactivation on graphenic nanocarbon surfaces

This article reports a direct chemical pathway for antioxidant deactivation on the surfaces of carbon nanomaterials. In the absence of cells, carbon nanotubes are shown to deplete the key physiological antioxidant glutathione (GSH) in a reaction involving dissolved dioxygen that yields the oxidized dimer, GSSG, as the primary product. In both chemical and electrochemical experiments, oxygen is only consumed at a significant steady-state rate in the presence of both nanotubes and GSH. GSH deactivation occurs for single- and multi-walled nanotubes, graphene oxide, nanohorns, and carbon black at varying rates that are characteristic of the material. The GSH depletion rates can be partially unified by surface area normalization, are accelerated by nitrogen doping, and suppressed by defect annealing or addition of proteins or surfactants. It is proposed that dioxygen reacts with active sites on graphenic carbon surfaces to produce surface-bound oxygen intermediates that react heterogeneously with glutathione to restore the carbon surface and complete a catalytic cycle. The direct catalytic reaction between nanomaterial surfaces and antioxidants may contribute to oxidative stress pathways in nanotoxicity, and the dependence on surface area and structural defects suggest strategies for safe material design.     

Growth and analysis of C nanotubes on ceramic polymer-additives

C nanotubes are synthesized by catalytic route on ceramic supports (Al2O3, MgO and CaO), usually utilized for polymer reinforcing/flame-retardancy, aiming at nanotube-based hybrid preparation. Chemical vapor deposition is carried out in i-C4H10+H2 atmosphere over 17 wt% Fe-catalysts upon different conditions. In order to clarify the influence of support material, calcination (450 degrees C or 750 degrees C) and reduction temperature (500 degrees C or 600 degrees C) of the catalysts, and synthesis temperature (600 degrees C or 700 degrees C), catalysts utilized and nanotubes obtained are systematically investigated by the use of several analysis techniques (electron microscopy, X-ray diffraction, thermo-gravimetry and Raman spectroscopy). The results obtained show that, in the considered range of variation, support material is the most influential parameter. The most catalytically active alumina supports allow achieving higher yields, but involve larger metallic inclusions and lower crystalline quality. Remaining supports behave oppositely. The reasons for such differences are discussed in the light of the current assessments on the nanotube growth and the results obtained are compared with those available in literature for similar catalysts.     

Efficient growth of horizontally aligned single-walled carbon nanotubes by chemical vapor deposition over MgO-supported bimetallic co-based catalysts

A series of supported cobalt-based catalysts (Co-X, where X = Mn, V, W, Gd, Mo) has been investigated for the growth of single-walled carbon nanotubes via catalytic decomposition of CH4. At 850 degrees C, Mn, W and Gd promoted Co-catalysts produce SWNTs while Mo and V do not yield any SWNTs. Furthermore, Co-Gd catalysts produce high quality SWNTs with very few defects, and also narrow diameter and chirality distributions (approximately 1 nm). The growth of horizontally aligned SWNTs (approximately 1 mm long) using the Co-Gd catalyst is also demonstrated.     

Few-layer nano-graphene structures with large surface areas synthesized on a multifunctional Fe:Mo:MgO catalyst system

We present the synthesis of nano-graphene structures with large surface areas and high purity over a high-yield Fe:Mo:MgO catalytic system. Two different hydrocarbon sources, acetylene and methane, were used, and their role in determining the size and morphology of the few-layer graphene sheets was studied. In addition, varying the active metal loading of the catalyst system influenced the formation and type of the resulting carbon nano-structures, e.g., carbon nanotubes or few-layer graphene. Growth of nano-graphene sheets was detected after only 5-min reaction time over this multifunctional catalytic system. High purity and crystalline graphene structures were synthesized indicating another advantage of using this particular catalyst system. This catalytic chemical vapor deposition can be scaled up for large-scale few-layer graphene production.     

Growth of metal-free carbon nanotubes on glass substrate with an amorphous carbon catalyst layer

We have investigated the direct growth of metal-free carbon nanotubes (CNTs) on glass substrates with microwave-plasma enhanced chemical vapor deposition (MPECVD). Amorphous carbon (a-C) films were used as a catalyst layer to grow metal-free CNTs. The a-C films were deposited on Corning glass substrates using RF magnetron sputtering with the use of a carbon target (99.99%) at room temperature. They were pretreated with hydrogen plasma using a microwave PECVD at 600 degrees C. Then, CNTs were prepared using microwave PECVD with a mixture of methane (CH4) and hydrogen (H2) gases. The CNTs were grown at different substrate temperatures (400 degrees C, 500 degrees C, and 600 degrees C) for 30 minutes. Other conditions were fixed. The growth trends of CNTs against substrate temperature were observed by field emission scanning electron microscopy (FE-SEM). The structure of a-C catalyst layer and grown CNTs were measured by Raman spectroscopy. High-resolution transmission electron microscopy (HR-TEM) images showed that the CNTs had bamboo-like multi-walled structures. Energy dispersive spectroscopy (EDS) measurements confirmed that the CNTs consisted of only carbon.     

High yield of pure multiwalled carbon nanotubes from the catalytic decomposition of acetylene on in-situ formed cobalt nanoparticles

For the first time, multiwalled carbon nanotubes (MWNTs) could be formed selectively in a high yield, free of any disordered carbon by-product, from the catalytic decomposition of acetylene at 600 degrees C on a CoxMg(1-x)O solid solution. Starting from 1 g of catalytic substrate, 4 g of pure MWNTs were obtained after its dissolution in boiling concentrated HCl, without any additional purification in strongly oxidizing medium, as is required for other methods of nanotube production. In situ reduction of CoO by dihydrogen liberated from acetylene decomposition allows highly divided metal particles to be continuously produced as synthesis proceeds. This is undoubtedly the reason for the good performance of the catalyst and for the ability to produce nanotubes in a narrow diameter range, namely from 10 to 15 nm. With the use of acetylene instead of methane, the synthesis proceeds at low temperature, which prevents the growth of carbon shells, in which the metal particles are generally embedded, decreasing their activity. Because of the very low specific surface area of the catalyst support, the amount of disordered carbon by-product formed is negligible.     

Carbon nanotubes by electrospinning with a polyelectrolyte and vapor deposition polymerization

Electrospinning of a polyelectrolyte and vapor deposition polymerization were combined to fabricate nanotubes of oxidatively stabilized poly(acrylonitrile) (PANDelta) with an outer diameter of 100 nm, a wall thickness of 14 nm, and centimeter-scale length. Poly(styrene sulfonate) sodium (PSSNa) nanofibers serves as sacrificial cores while vapor deposition polymerization was used to form smooth PAN sheaths of even thickness. After the PAN sheaths had been oxidatively stabilized, the PSSNa cores were etched away with water to form nanotubes of PANDelta. High-temperature carbonization of these nanotubes at 900 degrees C under Ar flow yielded carbon nanotubes with an outer diameter of 80 nm and wall thickness of 10 nm. Raman spectroscopy confirms that the carbon nanotubes were composed of highly disordered graphene sheets, consistent with the carbonization of PAN under similar conditions. These carbon nanotubes have many promising applications as catalyst supports, gas absorbents, and as encapsulants for controlled release of active compounds.     

Application of Ni:SiO2 nanocomposite to control the carbon deposition on the carbon dioxide reforming of methane

Stable Ni nanoparticles embedded in a mesoporous silica material were used as catalysts for the conversion of methane into synthesis gas. This catalyst has the singular properties of controlling the carbon deposition and deactivation of active sites. A comparative study of our nanocomposites with conventional catalysts showed that impregnation material presented a preferential encapsulation and growth of carbon nanotubes on the metal surface. The impregnated catalyst showed a higher tendency for carbon nanotube and whiskers formation.     

Synthesis of multiwalled carbon nanotubes from polyethylene waste to enhance the rheological behavior of lubricating grease

Abstract

Carbon nanotubes (CNTs) are attracting great interest in enhancing rheological behavior and thermal performance of lubricating grease. In this study, CNTs were synthesized by catalytic chemical vapor deposition (CCVD) method using low-density polyethylene (LDPE) waste as a cheap carbon source and Co/MgO as an effective catalyst. The effect of temperature on the catalytic pyrolysis of LDPE to produce CNTs has been studied. Catalytic pyrolysis of LDPE waste was conducted in a temperature range of 350–600?°C using the H-ZSM-5 catalyst. The structure and quality of CNTs were fully characterized using HR-TEM, XRD, and Raman spectroscopy. On the other hand, various concentrations of CNTs (0.2, 0.4, 0.6, 0.8, and 1.0?wt%) were mixed with pure lithium grease to determine the optimum percentage that improves the properties of nano-grease. The results showed that a high yield of multiwalled carbon nanotubes (MWCNTs) was obtained with high quality at temperatures ranging from 400 to 550?°C. Also, the addition of CNTs enhanced the rheological behavior of lithium grease, and the optimum percentage of CNTs was 0.8?wt%. Furthermore, the apparent viscosity and shear stress of lithium nano-grease increased by increasing the concentration of CNTs up to 0.8%. At this concentration, the penetration value of lithium nano-grease was greater than pure grease, and the dropping point increased by 12.5%. These results suggested that CNTs prepared from LDPE waste were an excellent additive to enhance the physicochemical properties of lithium grease.     

催化化学气相沉积法制备螺旋形多壁碳纳米管(英文)

以乙炔为碳源、FeMo/MgO催化剂为模板,采用催化化学沉积法制备了螺旋状多壁碳纳米管(hs-MWC-NTs)。其中FeMo/MgO模板,由作为发泡和助燃剂的柠檬酸燃烧而制成。FeMo/MgO催化剂的XRD谱图揭示其具有微晶的通性。应用SEM、TEM和Raman光谱剖析了合成的炭材料。SEM和TEM观察表明获得了hs-MWC-NTs;Raman光谱的D峰和G峰确认了所获碳纳米管(CNTs)的结晶状态。结果表明:此法乃是合成直径10nm～20nm螺旋形多壁碳纳米管的最容易和简便方法。     

CO-Mo/MgO催化剂上甲烷化学气相沉积制备高纯度多壁碳纳米管

以甲烷为碳源,co-Mo/MgO为催化剂,通过气相化学沉积制备了直径均匀的多壁碳纳米管(MWC-NTs).采用溶胶-凝胶法所制双金属催化剂的组成为Co∶Mo∶MgO=5∶20∶75(质量比).热重分析表明多壁碳纳米管产率高达313.67%.催化剂对于多壁碳纳米管生长的选择性是91.17%(其余为无定形碳).透射电子显微镜分析显示:催化剂七生长的MWCNTs平均直径为6.2±0.5nm(平均±标准偏差).通过稀酸的简单纯化处理,纯化样品的催化剂残存率降至0.72%.     

The role of the gas species on the formation of carbon nanotubes during thermal chemical vapour deposition

In this paper, we investigate the several roles that hydrogen plays in the catalytic growth of carbon nanotubes from the point of view of gas species, catalyst activation and subsequent interaction with the carbon nanotubes. Carbon nanotubes and nanofibres were grown by thermal chemical vapour deposition, using methane and a mixture of hydrogen and helium, for a range of growth temperatures and pre-treatment procedures. Long, straight carbon nanotubes were obtained at 900?°C, and although the growth yield increases with the growth temperature, the growth shifts from nanotubes to nanofibres. By introducing a helium purge as part of the pre-treatment procedure, we change the gas chemistry by altering the hydrogen concentration in the initial reaction stage. This simple change in the process resulted in a clear difference in the yield and the structure of the carbon nanofibres produced. We find that the hydrogen concentration in the initial reaction stage significantly affects the morphology of carbon fibres. Although hydrogen keeps the catalyst activated and increases the yield, it prevents the formation of graphitic nanotubes.     

Negative temperature coefficient of single-walled carbon nanotube-gold nanoparticle hybrid structures

Single-walled carbon nanotubes (SWNTs) were grown on gold nanoparticle (GNP) coated quartz substrates by alcohol catalytic chemical vapor deposition. The GNP coated substrates were coated with Co catalyst by a dip-coat method. The growth was then carried out at 800 degrees C under a pressure of 10 Torr in an atmosphere of ethanol vapor for 30 min. Characterizations have shown larger SWNT diameters with higher negative temperature coefficients for GNP coated substrates as compared to those of quartz substrates without GNPs. It is attributed that SWNT-GNP hybrid structures have a higher fraction of semiconductor-type pathways.     

Generation of single-walled carbon nanotubes from alcohol and generation mechanism by molecular dynamics simulations

Recent advances in high-purity and high-yield catalytic chemical vapor deposition (CVD) generation of single-walled carbon nanotubes (SWNTs) from alcohol are comprehensively presented and discussed on the basis of results obtained from both experimental and numerical investigations. We have uniquely adopted alcohol as a carbon feedstock, and this has resulted in high-quality, low-temperature synthesis of SWNTs. This technique can produce SWNTs even at a very low temperature of 550 degrees C, which is about 300 degrees C lower than the conventional CVD methods in which methane or acetylene is typically used. We demonstrate the excellence of the proposed alcohol catalytic CVD method for high-yield production of SWNTs when Fe-Co on USY-zeolite powder was used as a catalyst. At optimum CVD conditions, a SWNT yield of more than 40 wt % was achieved over the weight of the catalytic powder within the reaction time of 120 min. In addition to the advantages for mass production, this method is also suitable for the direct synthesis of high-quality SWNTs on Si and quartz substrates when combined with the newly developed liquid-based "dip-coat" technique to mount catalytic metals on the surface of substrates. This method allows easy and costless loading of catalytic metals without the need for any support or underlayer materials that were usually required in previous studies for the generation of a sufficient quantity of SWNTs on an Si surface. Finally, the result of molecular dynamics simulation for the SWNT growth process is presented to obtain a fundamental insight into the initial growth mechanism on the catalytic particles.     

Growth of double-walled carbon nanotubes using a conditioning catalyst

Here we describe the effect of different synthetic conditions on the quality and purity of double-walled carbon nanotubes (DWNTs) with the aid of a conditioning catalyst. By lowering the reaction temperature down to 875 degrees C and utilizing a conditioning catalyst, increased purity and a decreased inner diameter of the DWNTs was achieved, while adverse results were observed with increasing reaction temperature. Based on detailed high-resolution transmission electron microscopy studies on the diameter distribution of the tubes, preferential growth conditions for DWNTs over single-wall carbon nanotubes are identified solely from increased carbon solubility considerations (caused by an increased portion of active carbon species by use of Mo) for the same distribution of metal particles.